Increasingly high requirements are placed on homogenizing and dispersing media of the same or different aggregate states as a precondition of a large number of process steps in chemical or engineering technology, said requirements having to be met with the aid of generally complex static or dynamic mixing systems.
During horizontal drilling, too, there is the requirement to mix a liquid with a powdered substance or a liquid or a suspension if, in order to facilitate and improve the drilling operation, for example a bentonite-water suspension is to be used as a drilling or flushing liquid. Such a suspension keeps the drilling dust in suspension, lubricates the pipe string as it is pulled in and protects the latter against the surrounding earth after a certain hardening phase. In order to vary the characteristics of the suspension, additives, such as soda ash or polymers, can be added.
Drilling liquids are normally mixed in a separate storage tank by means of a stirrer operating in this tank, that is to say a dynamic mixer, or by means of a high-speed pump.
These mixing systems have an increased requirement for space and lead to time delays in the drilling operation if, after a batch of drilling liquid has been used, a new batch has to be prepared. They do not permit a compact design of the overall drilling system.
Static mixing systems are also known which, as opposed to dynamic systems, do not have any stirrer and require less space.
The use of static mixers in mixing systems for the production of drilling liquid for horizontal drilling methods is known from German Patent Application 199 18 775.4. In the method described therein for the production of a drilling liquid, the added medium, for example bentonite, is led to the water in powder form upstream or downstream of a hydraulic pump that transports the drilling liquid to the drilling system A static mixing section, which homogenizes the added substance and the water, can be arranged downstream of the pump.
A static mixer, as known for example from “wägen+dosieren” (weighing and metering) 3/1997, pages 23 to 26, generally comprises a plurality of different types of individual mixer elements which are connected one after another and can be inserted into a feed line or discharge line system with the aid of an adapter. Each of these mixer elements has one or more deflection surfaces which, if necessary, are penetrated by one or more passages. The deflection surfaces following one another either within a mixer element or in mixer elements connected downstream are in this case always inclined at small angles with respect to one another and likewise, coincident with the flow direction of the medium flowing in the line, have a small angle of inclination that differs from 90°.
The deflection surfaces, which are at a particular axial angle in relation to one another and to the flow direction, produce forcible guidance of the flow, so that its flow direction rotates repeatedly. The passages which may penetrate the deflection surfaces likewise run at angles to one another and to the deflection surfaces so that both the flow is divided up and a repeated change in the flow direction takes place. The individual streams are guided together again at other deflection surfaces.
This repeated division, deflection and guiding together of the media has the effect of its homogenization or dispersion.
The selection of different mixer geometries is made as a function of the Reynolds number which, as the quotient of the inertial forces and the frictional forces, depends, amongst other things, on the material characteristics of the media. At a critical flow velocity, the inertial forces exceed a characteristic value, as compared with the frictional forces, so that the flow becomes turbulent.
Furthermore, the selection of the mixer geometries and the size of the overall mixing system, that is to say the number of mixer elements connected one after another, is made as a function of the permissible pressure loss in the flow, which primarily has to be assessed in view of the critical velocity required for the turbulence and the requirements of the process steps which follow.
Furthermore, the geometry of the deflection surfaces and passage openings and their arrangement relative to one another and to the flow direction have to be arranged in such a way that, as far as possible, the absence of dead zones can be ensured, since these prevent homogeneous mixing.
A considerable disadvantage of the known static mixers resides in the fact that the mixer elements, produced with complex geometry, have to be produced in complicated production processes, which give rise to a considerable expenditure in time and cost. Above all, the partly solid configuration of the mixers with differently aligned passages makes a high expenditure on material necessary.
A further disadvantage of known mixers is that cleaning of the mixers is made considerably more difficult, because of the deflection surfaces being at changing angles to one another Reliable, simple cleaning, for example by means of a cleaning fluid merely flowing through the mixer, is inadequate.